1. Field of the Invention
The present invention relates generally to communication channels and systems for transmitting data. In particular, the present invention relates to a system and method for transmitting compressed digital video signals over a communication channel. Still more particularly, the present invention relates to a system and method for transmitting compressed digital video signals over digital subscriber loop (DSL) access networks and asynchronous transfer mode (ATM) networks.
2. Description of the Background Art
There are presently a variety of different communication channels for transmitting or transporting video data. For example, communication channels such as digital subscriber loop (DSL) access networks, ATM networks, satellite, or wireless digital transmission facilities are all well known. In fact, many standards have been developed for transmitting data on the communication channels. The present invention relates to such communication channels, and for the purposes of the present application a channel is defined broadly as a connection facility to convey properly formatted digital information from one point to another. A channel includes some or all of the following elements: 1) physical devices that generate and receive the signals (modulator/demodulator); 2) physical medium that carries the actual signals; 3) mathematical schemes used to encode and decode the signals; 4) proper communication protocols used to establish, maintain and manage the connection created by the channel. The concept of a channel includes but is not limited to a physical channel, but also logical connections established on top of different network protocols, such as xDSL, ATM, TCP/IP, wireless, HFC, coaxial cable, etc.
The channel is used to transport a bit stream, or a continuous sequence of binary bits used to digitally represent compressed video, audio or data. The bit rate is the number of bits per second that the channel is able to transport. The bit error rate is the statistical ratio between the number of bits in error due to transmission and the total number of bits transmitted. The channel capacity is the maximum bit rate at which a given channel can convey digital information with a bit error rate no more than a given value. And finally, a multiplex is a scheme used to combine bit stream representations of different signals, such as audio, video, or data, into a single bit stream representation.
One problem with existing communication channels is their ability to handle the transportation of video data. Video data is much larger than many other types of data, and therefore, requires much more bandwidth from the communication channels. Since transmission of video data with existing communication channels would require excessive amounts of time, compression is an approach that has been used to make digital video images more transportable.
Digital video compression schemes allow digitized video frames to be represented digitally in much more efficient manner. Compression of digital video makes it practical to transmit the compressed signal by digital channels at a fraction of the bandwidth required to transmit the original signal without compression. International standards have been created on video compression schemes. These include MPEG-1, MPEG-2, H.261, H.262, H.263, etc. These standardized compression schemes mostly rely on several key algorithm schemes: motion compensated transform coding (for example, DCT transforms or wavelet/sub-band transforms), quantization of the transform coefficients, and variable length encoding (VLC). The motion compensated encoding removes the temporally redundant information inherent in video sequences. The transform coding enables orthogonal spatial frequency representation of spatial domain video signals. Quantization of the transformed coefficients reduces the number of levels required to represent a given digitized video sample and is the major factor in bit usage reduction in the compression process. The other factor contributing to the compression is the use of variable length coding (VLC) so that most frequently used symbols are represented by the shortest code word. In general, the number of bits used to represent a given image determines the quality of the decoded picture. The more bits used to represent a given image, the better the image quality. The system that is used to compress digitized video sequence using the above described schemes is called an encoder or encoding system.
In the prior art compression schemes, the quantization scheme is lossy, or irreversible process. Specifically, it results in loss of video textural information that cannot be recovered by further processing at a later stage. In addition, the quantization process has direct effect on the resulting bit usage and decoded video quality of the compressed bit stream. The schemes at which the quantization parameters are adjusted control the resulting bit rate of the compressed bit stream. The resulting bit stream can have either constant bit rate, CBR, or variable bit rate, VBR. CBR compressed bit stream can be transmitted over channel delivers digital information at a constant bit rate.
A compressed video bit stream generally is intended for real-time decoded playback at a different time or location. The decoded real-time playback must be done at 30 frames per second for NTSC standard video and 25 frames per second for PAL standard video. This implies that all of the information required to represent a digital picture must be delivered to the destination in time for decoding and display in timely manner. Therefore, this requires that the channel must be capable of making such delivery. From a different perspective, the transmission channel imposes bit rate constraint on the compressed bit stream. In general, the quantization in the encoding process is adjusted so that the resulting bit rate can be accepted by the transmission channel.
Because both temporal and spatial redundancies are removed by the compression schemes and because of variable length encoding, the resulting bit stream is much more sensitive to bit errors or bit losses in the transmission process than if the uncompressed video is transmitted. In other words, minor bit error or loss of data in compressed bit stream typically results in major loss of video quality or even complete shutdown of operation of the digital receiver/decoder.
Further, a real-time multimedia bit stream is highly sensitive to delays. A compressed video bit stream, when transmitted under excessive and jittery delays, will cause the real-time decoder buffer to under flow or overflow, causing the decoded video sequence to be jerky, or causing the audio video signals out of synchronization. Another consequence of the real-time nature of compressed video decoding is that lost compressed data will not be re-transmitted.
One particular communication channel that is becoming increasingly common is Asynchronous Transfer Mode (ATM) networks. ATM networks are based on the ATM transport protocol which can be used to transport data or multimedia bit stream with pre-specified quality of service. Unlike the xDSL standard, ATM protocols specifies how data is first packetized into fixed sized data units, call cells. It also specifies how such a cell stream can be multiplexed, de-multiplexed, switched and routed between different locations to support end-to-end connections at given bit rate and/or quality of service (QOS). In ATM networks, data bit stream to be transported are first converted into fixed sized ATM cells, each cell has a 5 byte header and up to 48 bytes of payload. Of particular interests to our invention is the capability of ATM networks to carry MPEG transport streams.
In ATM networks, connections can be established with pre-determined grade of QOS. Conditions of network utilization, along with call admission control sometimes may prevent new connection being established at the given bit rate or given quality of service. In such cases, the requested connection may either have to be rejected or a new set of admissible connection parameters have to be negotiated between the network service provider and the user requesting the connection.
ATM networks can be used to carry either constant bit rate (CBR) or variable bit rate (VBR) bit stream. The bit stream may be compressed bit stream or data. In either case, an agreement must be made between the user requesting the connection and the network service provider. The connection agreement includes the bit rate profile of the bit stream and quality of service. If such an agreement cannot be reached, either the connection must be rejected, or the bit rate profile must be modified, or the mismatched bit rates may cause periodic loss of data within the ATM networks, which results in corrupted bit stream in the receiver/decoder. Specific decoded video quality depends on the decoder design.
The process of modifying the bit rate profile of the input bit stream is called traffic shaping. The objective of the traffic shaping is to smooth out the burstiness of the ATM cell stream so that the resulting bit rate profile is more constant than that without traffic shaping. Existing methods to do traffic shaping include using a large buffer to smooth out the bit rate fluctuation, or to selectively drop ATM cells during high bit rate period. These methods were originally designed for connections carrying data streams or non-real-time multimedia bit stream. In the case of using large buffers, data bit stream is not sensitive to large delay variations. In the case of selective cell dropping, higher layer communication protocols will retransmit the lost information at a later time. However, in the case of transporting real-time compressed video, none of the method is acceptable because of the tight constraint on the end-to-end delay variation. For example, when ATM networks are used to transport MPEG-2 transport stream, the end-to-end jitter typically shall not be more than 1 millisecond. The use of rate converter as traffic shaper will solve exactly this problem.
Within ATM networks data loss may also occur when there is not enough channel bandwidth to send all of the ATM cells. In this case, ATM cells may be dropped from the overflowing buffers. If the bit stream carried in the connection complies with the connection agreement, such event will occur very infrequently and falls within the bounds of the quality of service constraints.
Also, it is important to point out that in general compressed video bit stream are generated by either real-time encoders or pre-compressed video server storage systems, which are likely to be in a remote site, away from the network facility itself. This implies that in general it is difficult to encode the video signal with a resulting bit rate profile tailored to the connection bandwidth available from the ATM network.
ATM network protocols are under intense international standardization effort and several industry wide inter-operable specifications have been produced, including the one specific on means to carry MPEG-2 transport streams over ATM networks using ATM Adaptation Layer 5 (AAL-5).
Therefore, there is a need for a system and method for transmitting digital video information over data channels, that is simple to implement and has smaller delay, and can match the input bit rate to the channel bandwidth, resulting in a dramatically reduced bit error rate.